Fungicide is best, but...
I doubt that it would help much--but potassium is available as muriate of potash 0-0-62, at any farm elevator. Apply as dry, but it has a high burn potential. Or apply as liquid dissolved in water; don't burn the grass; no more than 50 pounds in 200 gallons of water.

Or perhaps a high potash formula like Everris Osmocote Bloom 12-7-18--premium type--and price.

In the long haul, an over seed with a higher quality more disease-resistant grass is probably the best bet.